This invention relates to radiation detectors for detecting ionizing radiation, such as X-ray and gamma radiation. More specifically, this invention relates to multi-cell X-ray detectors of the type used in medical diagnostic apparatus, such as computerized tomography (CT) scanners.
U.S. Pat. No. Re. 30,644, issued to Whetten, et al, and U.S. Pat. No. 4,119,853, issued to Shelley et al (both of which are assigned to the same assignee as the present invention and both of which are incorporated herein by reference) disclose and claim multi-cellular X-ray detectors of the type suitable for use with CT scanners. Generally, the detector is composed of a plurality of detector cells defined by radiation-opaque electrode plates supported between a pair of parallel ceramic substrates which are in turn secured to upper and lower metallic frame members of a detector-array assembly. The detector is positioned within a detector housing which contains a pressurized ionizable gas, such as xenon. In operation, alternate ones of the electrode plates (bias electrodes) are connected to a source of electric potential of about 500 volts, while the remaining electrodes (signal electrodes) are maintained at ground potential. X-ray or gamma radiation, intensity modulated by passage through an object undergoing examination, entering a detector cell ionizes the xenon gas to create photoelectron/ion pairs which are collected by the signal electrode inducing therein a current proportional to the intensity of the ionizing radiation. The current thusly induced in each signal electrode is coupled to external instrumentation, including digital computer means, for processing in a well-known manner to reconstruct transverse images of the object.
To obtain good image quality in the reconstructed images, particularly in computerized tomography, the detector must measure X-ray photons efficiently and with a high degree of resolution. Good spatial resolution is obtained by spacing the electrode plates closely and uniformly over the entire length of the detector. It is also important for each cell to have identical and stable detecting characteristics. Ideally, the output signal from a detector cell should comprise only the signal due to excitation by ionizing radiation. In practice, however, a spurious output signal is frequently superimposed on the desired signal. This spurious signal, generally referred to as microphonic noise, can be traced to the construction of the detector. The electrodes are fabricated from thin metal plates positioned in close proximity to one another with a relatively large potential difference between them. Mechanical vibrations transmitted to the plates may significantly vary the capacitance between electrodes and, thus, introduce spurious currents in the signal electrodes. These spurious currents are generated during active scans and are detected along with the desired currents by the current-sensing electronics, causing errors in the X-ray-intensity measurements. Although the spurious currents are small (on the order of picoamperes), they are nevertheless significant compared to the X-ray-induced signals and manifest themselves as image-degrading artifacts in the reconstructed images.
It is, therefore, an object of the present invention to provide a multi-cellular, ionizing radiation detector with significantly reduced sensitivity to mechanical vibrations.
It is another object of the invention to reduce detector sensitivity to microphonic noise by means which is easily removable and replaceable and which is readily adaptable to a variety of detector designs.
It is still another object of the invention to reduce detector sensitivity to mechanical vibrations by means which is easily replaceable, allowing changes in the design thereof to match changes in the detector parameters with minimum impact on image quality.
It is a further object of the invention to reduce the sensitivity of the detector to interference from microphonic noise by means which is easily replaceable so as to allow its replacement with minimum impact on detector design and manufacturability.